1. Field of the Invention
This invention relates generally to open-loop control and closed-loop control for active vehicle rear-wheel steering and, more particularly, to open-loop control and closed-loop control for active rear-wheel steering, where both the open-loop control and the closed-loop control employ vehicle parameter estimates to compensate for changes in vehicle parameters.
2. Discussion of the Related Art
It is known in the art to employ automatic rear-wheel vehicle steering based on vehicle dynamic information during a vehicle turn, or yaw, to enhance the vehicle stability. Active wheel steering control of a vehicle can improve vehicle stability over a conventional vehicle having only steerable front wheels. The rear-wheel steering assist can be in-phase steering or out-of-phase steering. In-phase rear wheel steering steers the rear wheels in the same direction as the front wheels, and is typically provided at higher vehicle speeds. Out-of-phase rear wheel steering steers the rear wheels in an opposite direction as the front wheels to provide a tighter turning radius, and is typically provided at lower vehicle speeds.
Open-loop automatic rear-wheel steering provides a certain amount of rear-wheel steering assist depending on the amount of front-wheel steering provided by the vehicle operator. In other words, a predetermined function is used to turn the rear wheels a certain amount, either in-phase or out-of-phase, in response to the turning of the front wheels at a particular vehicle speed. It is known to provide closed-loop automatic rear-wheel steering based on feedback in the event that the vehicle is not following the steering path commanded by the vehicle operator. For example, slippery road conditions may prevent the vehicle from turning in the desired direction because the wheels may slip along the road surface. Further, the vehicle's rear quarter may “fish-tail,” also providing a different turn angle than was intended. Closed-loop rear-wheel steering assist systems sense the actual vehicle yaw rate and the commanded yaw rate, and generate a gain signal that provides the steering assist by the rear wheels if the vehicle yaw rate and the commanded yaw rate are not the same. In these types of active rear-wheel steering control systems, the open-loop control is always active and the closed-loop control is only activated if a potential handling and stability problem is detected.
Vehicles are designed so that the vehicle handling response complies with certain design specifications. Vehicle dynamic parameters define the vehicle handling response, where nominal parameters define a nominal vehicle handling response. The vehicle dynamic parameters of understeer coefficient, front cornering compliance and rear cornering compliance are the most dominant dynamic vehicle parameters for determining the stability and dynamic handling behavior of a vehicle. The understeer coefficient defines the vehicle yaw rate or turning radius for a particular steering angle. The front cornering compliance and the rear cornering compliance define the distribution of the vehicle side-slip to the front and rear axles when the vehicle is turning. The cornering compliances include the ratio defined by the lateral slip angle and the lateral force of the wheels. These parameters vary according to different vehicle loading, tire pressure, tire wear, and vehicle-to-vehicle variations of suspension characteristics, etc.
The control gains for rear-wheel steering control systems, including both open-loop and closed-loop control, are optimally determined based on “out-of-factory” vehicle dynamic parameters. During operation of the vehicle over its lifetime, however, the factory-tuned vehicle parameters change as a result of short-term vehicle variations, such as vehicle loading and tire pressure changes, and long-term variations, such as tire wear and vehicle suspension age. When the vehicle dynamic parameters change from their original settings, the original steering control gains are no longer optimal, resulting in a different vehicle handling feel and vehicle stability degradation.
U.S. patent application Ser. No. 10/863,956, titled Real-Time Vehicle Dynamics Estimation System, filed Jun. 9, 2004, assigned to the assignee of this application and herein incorporated by reference, discloses a system that determines vehicle parameter estimates as a vehicle ages to provide an indication of a potential problem. It would be desirable to use these estimated dynamic parameters in an open-loop and closed-loop rear-wheel active steering system to improve the closed-loop steering control gains and the open-loop steering gain as the vehicle ages and is subjected to different conditions. By modifying the open-loop and closed-loop gain schedule in response to estimated vehicle dynamic parameters, the handling feel and directional stability of the original vehicle can be maintained over the lifetime of the vehicle.